The present invention relates to electronics, and in particular, to a low-noise amplifier (LNA) used in radio-frequency signal tuners intended for mobile telephone circuits that operate in separate frequency bands. For example, the receive bands of the GSM standard are in the range of 925 to 960 MHz and the receive band of the DCS standard is in the range of 1,805 to 1,880 MHz. The center frequency associated with the receive band in the UMTS standard is 2,200 MHz and for the Bluetooth standard is 2,400 MHz.
The amplifiers of mobile telephones operating in accordance with a predetermined reception standard include two cascode-connected amplifier stages and an LC (inductor/capacitor) circuit which has a maximum impedance at a given frequency, in this instance the center frequency of the receive band. The inductance of the circuit has a high Q which produces a relatively narrow gain/frequency curve and a noise figure yielding a very low ratio between output noise and input noise in the receive band.
There is a need to design cellular mobile telephones that operate in accordance with more than one transmission standard, for example multistandard telephones which can operate in accordance with all standards including the DCS standard and the Bluetooth standard. It is therefore necessary to modify the low-noise amplifier stage connected to the receive antenna of the telephone.
A first approach is to use a number of LNAs associated with respective different receive frequency bands (corresponding to the various standards) that the mobile telephone is to be able to receive. The various amplifiers are associated with control means which switch the signal from the antenna to one of the amplifiers according to the receive band selected by the telephone. This approach obviously has a major drawback in terms of overall size and complexity of implementation.
Another approach includes using a degenerated inductive load, i.e., an inductive component having a lower Q. This widens the gain/frequency curve, and signals in separate receive frequency bands can therefore be amplified. However, the gain of this amplifier structure is reduced, and, more seriously, the noise figure is degraded, i.e., the ratio between output noise and input noise is higher. This degraded noise figure is incompatible with some transmission standards, in particular the UMTS standard.
In view of the foregoing background, an object of the present invention is to provide a wide-band, low-noise amplifier (LNA) that operates in accordance with more than one transmission standard, and has a high gain throughout the range of use of the amplifier, combined with a low ratio between output noise and input noise in the same range of use, i.e., regardless of the frequency band selected within that range.
Another object of the invention is to provide a wide-band, low-noise amplifier that is easy to implement in the form of an integrated circuit and with an overall size that does not represent a penalty.
These and other objects, advantages and features of the present invention are provided by an amplifier that includes an input amplifier stage, an output amplifier stage cascode-connected with the input amplifier stage, and a load stage connected to the output stage.
According to a general feature of the invention the load stage includes a plurality of circuits each including a capacitive component and an inductive component having a Q greater than 10 and having respective different resonant frequencies. The gain curves respectively associated with the circuits have, to within a stated tolerance (for example, approximately xc2x11 dB), the same maximum gain value at the resonant frequencies. The gain curves respectively associated with two circuits having respective immediately adjacent resonant frequencies overlap below a threshold 3 dB, to within a stated tolerance (for example, approximately xc2x11 dB), below the maximum gain value.
This produces an amplifier which can operate at all frequencies from the lowest resonant frequency of the load stage to its highest resonant frequency. This type of structure works if the resonant frequencies are very close together to create a xe2x80x9cflatxe2x80x9d gain over the range of frequencies to be covered. The flat gain and the close spacing of the resonant frequencies are obtained by overlapping the gain curves from a threshold 3 dB below the maximum gain value and by using the same maximum gain value for each LC circuit.
The above structure also yields a flat configuration for the noise figure, leading to a very low value of the output noise/input noise ratio throughout the frequency range to be covered. The number of circuits and their resonant frequencies are obviously chosen so that the various center frequencies correspond to the various standards that a mobile cellular telephone is to be able to accommodate, i.e., from the lowest resonant frequency to the highest resonant frequency.
The amplifier according to the invention is therefore a wide-band amplifier and must not be confused with a xe2x80x9cmulti-bandxe2x80x9d amplifier, i.e., an amplifier made up of a number of low-noise amplifiers associated with respective different receive center frequencies. The wide-band amplifier according to the invention is also distinguished from a two-band or even a three-band, or more generally a multi-band amplifier, as described in French Patent Application No. 9,911,032 which describes an LNA equipped with a load stage including a plurality of LC circuits whose resonant frequencies must be very widely spaced.
All the circuits of the load stage of the amplifier according to the invention can be connected either in parallel or in series. If all the circuits are connected in series with the output amplifier stage, all the circuits then have at their respective resonant frequency the same first impedance value to within a specified tolerance (for example, a tolerance of xc2x120%, which corresponds to the tolerance of xc2x11 dB referred to above). Furthermore, two circuits associated with two immediately adjacent resonant frequencies have at the median frequency between their respective resonant frequencies the same second impedance value, which is equal to half the first impedance value, again to within the specified tolerance, for example to within xc2x120%.
In other words, the flatness of the gain curve over the entire frequency range and the overlapping of two adjacent frequency bands is reflected in terms of impedance in what has just been referred to above for an embodiment in which the circuits are all connected in series with the output amplifier stage.
If all the circuits were connected in parallel, the second impedance value (i.e., that corresponding to the median frequency) would then be twice the first impedance value, to within the stated tolerance. However, an embodiment with the circuits connected in series is more advantageous in terms of overall size than an embodiment using a parallel connection. For a series connection, the inductances can have lower values, consequently leading to smaller dimensions on the silicon chip.
Although an amplifier according to the invention can be implemented with a single input, it is particularly advantageous for it to have a differential structure, to enable common mode rejection. Thus in a differential structure embodiment the input amplifier stage and the output amplifier stage each include a pair of transistors. The load stage then includes two identical groups of circuits and each of the two groups is connected to a respective transistor of the output amplifier stage.
The invention also provides an RF signal receiver, and in particular, a cellular mobile telephone including an amplifier as described above.